Various systems have been devised and implemented to control the change of gear ratios in an automatic transmission. This is generally done with a hydraulic system, including a hydro-mechanical control valve mechanism for modifying the pressure applied to various friction elements in the transmission and to selectively hold and release different components of a planetary gear set. For example, the engine output shaft can be connected to drive the sun gear of such a set, which also includes a plurality of planet gears rotatable on a carrier element and disposed between the sun gear and the outer ring gear. When the ring gear is held against rotation, output drive torque is available from the planet carrier at a first speed ratio. To change ratio, the sun gear is locked to the outer ring gear, which is no longer held against rotation, providing a direct drive connection. This is the operation which occurs in a typical upshift.
A great deal of research has been directed to optimizing the shift "quality". In general, it is not desirable to have a shift of very brief duration, as this produces a jerk by the rapid change in vehicle acceleration, which is very noticeable and found objectionable by most drivers. If the shift time is stretched out for too long a period, undue wear is imposed on the friction elements and other components of the transmission. Therefore, the optimum shift quality occurs somewhere between the too-short time duration, producing the objectionable jerk sensation, and the too-long time duration producing the objectionable component wear. A comprehensive treatment of this subject has been provided by F. J. Winchell and W. D. Route in "Ratio Changing the Passenger Car Automatic Transmission", which appears as Chapter 10 in the SAE publication "Design Practices--Passenger Car Automatic Transmissions", Copyright by the Society of Automotive Engineers, Inc., 1973. In particular FIG. 21 of this chapter depicts speed, torque and pressure variations during a power-on upshift.
A significant advance in ratio-changing control has been achieved in a control system which regulates the energization of the on-coming friction element, in which the off-going friction element is embodied in a two-way clutch which automatically releases. Hence, the control is directed only to the precise regulation of the on-coming element in a three member planetary gear set. This significant advance is described and claimed in the earlier application of Alan L. Miller and John S. Ivey entitled "Transmission Control System", filed Feb. 27, 1976, Ser. No. 661,896, and which is assigned to the assignee of this invention.
Such earlier systems generally use a controlled friction element in parallel with the one-way clutch, for reasons explained hereinafter. If the friction element can be precisely controlled, the one-way clutch can be eliminated. It is therefore a principal object of this invention to provide a more economical control system for shifting an automatic transmission, in which precise control is provided not only for the on-coming friction element but also for the off-going friction element, to obviate the need for a one-way clutch.